Four members of the gang that kidnapped, raped and murdered Mary-Ann Leneghan were on probation at the time of the killing, it emerged today.

Adrian Thomas, Michael Johnson, Jamaile Morally and Indrit Krasniqi were all under supervision when they abducted the 16-year-old and her friend from a car park in Reading in May last year.

News that the four gang members had been under supervision of the probation service emerged at the end of a two-month trial at Reading crown court.

Mary-Ann was stabbed to death and her friend shot and left for dead after a three-hour ordeal in which they were tortured, raped and sexually assaulted.

The home secretary, Charles Clarke, expressed his deep regret over the "tragic death" of Mary-Ann and acknowledged improvements needed to be made.

"It is always disturbing when a serious crime is committed by offenders under supervision in the community," he said.

"It is vital that we learn any lessons we can from these cases and take any steps necessary to ensure that offenders are properly managed and the public are properly protected."

Mr Clarke said lessons learned from the case would be included in changes to the assessment and management of offenders in the community set to be announced next month.

However, the Conservative shadow home secretary, David Davis, said the government had ignored repeated warnings about the failure of non-custodial sentences.

"It is about time the government realised the shocking consequences of their policy of allowing dangerous criminals out of prison and owned up to their complete mismanagement of the probation service," Mr Davis said.

Thomas, 20, Jamaile Morally, 22, his 23-year-old brother Joshua and Llewellyn Adams, 24, all from south London, were convicted at Reading crown court on Friday.

Johnson, 19, had already pleaded guilty while Krasniqi, 18, was convicted of murder today. The six will be sentenced next month.

Harry Fletcher, the assistant general secretary of the probation union Napo, confirmed the four men had been under probation supervision in south London.

"This is an appalling case, but the conditions of probation supervision appear to have been adhered to," he said. "The probation service has an extremely difficult job supervising thousands of dangerous offenders.

"Additional trained and experienced staff are absolutely essential to ensure the chances of a tragedy like this happening again are kept to an absolute minimum."

Mary Ann's family today asked to be allowed to grieve in peace following the convictions.

"Mary-Ann was a wonderful girl who was greatly loved by her friends and family," a family statement said. "She will be sorely missed. We are now left with the task of trying to piece our lives back together."

DS Mark Warwick paid tribute to the "strength of character" of the survivor of the attack, who gave evidence against the six men.

"There is no doubt in my mind that, without the first rate information provided by the witness, we would not be where we are today," he said.

After reaching their final verdict today, jurors were told of the murderers' catalogue of previous offences involving drugs and violence.

One member of the group, Johnson, had been sentenced to three and a half years in prison in 2001 for abducting a 13-year-old boy with learning difficulties and subjecting him to an ordeal in which he was hung upside down and beaten.

At the time of the attack, Johnson was only 13 years old, and the court heard he had since carried out a string of violent offences, including assaults on a fellow care home resident, two minicab drivers and a police officer.

Thomas, Krasniqi and Jamaile Morally also had convictions for drugs and weapons possession, common assault and handling stolen goods respectively.

Joshua Morally was not on probation at the time of the murder, but had convictions from six years ago for robbery and shoplifting. Adams was the only member of the gang with no criminal record.

Mary-Ann and her friend were abducted from a car park in the centre of Reading by the six men.

They were taken to a hotel, where they were raped, tortured and repeatedly told they would die.

The court heard Mary-Ann was taken into the hotel bathroom by five men, raped and sexually humiliated. Her friend was made to strip and perform sexual acts. They were forced to smoke crack cocaine and heroin.

The pair were then taken to Prospect Park, where Mary-Ann was stabbed 40 times. Her friend was shot in the head at close range but survived because the bullet did not pierce her skull.

During the trial, it emerged that the two girls could have been targeted because the men thought they were behind a burglary at one of their flats.

Richard Thomas QC, for the prosecution, said the girls had been asked during the attack whether they had stolen the gang's drugs. He said they were told: "How did you expect us to let you get away with it?".

The judge today exempted the jurors from jury service for ten years, telling them: "I appreciate how unpleasant and stressful this case has been for you to hear."

He adjourned sentencing until April 28 to allow pre-sentence reports to be compiled.